Multi-function airborne sensor system

ABSTRACT

Various embodiments provide a sensor system including a first optical sub-system having a first plurality of optical elements, and a second optical sub-system having a second plurality of optical elements including a first mirror. The second optical sub-system is configured to rotate about a first axis relative to the first optical sub-system and the first mirror is configured to rotate about a second axis substantially perpendicular to the first axis. The first axis and the second axis are arranged so as not to intersect each other so as to maximize a field of regard of the sensor system.

BACKGROUND

This disclosure pertains to optical sensor systems in general and inparticular to a multi-function airborne sensor system combining infraredsearch and track (IRST), targeting, and standoff reconnaissancefunctions in the same airborne sensor system.

Demand for imaging sensors that provide infrared search and track(IRST), targeting or standoff reconnaissance functions is increasing.These type of sensors can be used in various applications such as on anaircraft including an unmanned aerial vehicle (UAV) platform forstandoff reconnaissance or on a jet aircraft for IRST, targeting andstandoff reconnaissance.

IRST is often used for detecting and tracking objects which emitinfrared radiation such as jet aircrafts, helicopters, etc. Generally,IRST systems are passive in that they do not transmit or send anyradiation of their own unlike radar or Light Detection and Ranging(LIDAR) or Laser Detection and Ranging (LADAR). However, some IRSTsystems can incorporate laser rangefinders to provide information on anobject's position. IRST system are currently used in many aircrafts,particularly, fighter aircrafts to provide air superiority. As its nameindicates, an IRST system operates generally in the infrared wavelengthrange, but visible wavelength sensing capability is typically alsodesired within the IRST sensor.

Stand-off reconnaissance also known as “scouting” is used for survey orobservation to gain or collect image information which can be used forintelligence gathering. The wavelength ranges of interest for standoffreconnaissance include the visible wavelength range between about 0.4 μmand about 0.7 μm, near-infrared wavelength range between about 0.7 μmand about 1 μm, the short wavelength infrared radiation (SWIR) in thewavelength range between approximately 1 μm and 3 μm, mid wavelengthinfrared radiation (MWIR) in the wavelength range between approximately3 μm and 5 μm, and long wavelength infrared radiation (LWIR) in thewavelength range between approximately 8 μm and 12 μm.

Targeting, on the other hand, is used for target location anddesignation, for example, in fighter aircrafts and bombers foridentifying targets and guiding precision guided munitions such aslaser-guided bombs or missiles to designated targets. Some targetingsystems have a laser (e.g., an infrared laser) that can designate atarget for laser-guided munitions, enabling an aircraft carrying atargeting system to designate its own targets or designate targets forother friendly units. An additional active laser function that may alsobe implemented in a targeting sensor is laser rangefinding.

Currently, each of the IRST function, standoff reconnaissance functionand targeting function is provided as a separate sensor system.Therefore, a user desiring to utilize two or more of these functions isrequired to purchase two or more separate sensor systems which canincrease the overall cost of owning such separate sensor systems. Inaddition to the cost, providing two or more of such separate sensorsystem on a user platform increases the overall complexity. Thisseparation of the three sensing functions into three separate sensorsystems has generally been motivated by, among other things, thesignificantly different fields of regard required for each sensingfunction: a) the IRST field of regard is very wide in azimuth(horizontal), but generally forward looking, b) the standoffreconnaissance field of regard is wide in pitch (horizontal), butgenerally side looking, and c) the targeting field of regard is verywide in elevation, and can extend from many degrees above the localhorizon (forward) to many degrees past (behind) the local nadir(vertical) by as much as 60 deg.

In addition, none of these three separate sensor systems currently hasthe capability to incorporate advanced coherent LADAR subsystems such aslong range vibrometry. Prior attempts to add advanced LADAR capabilityto any of the IRST sensor system, standoff reconnaissance sensor systemand targeting system has met with limited success due to the fact thatadding such advanced LADAR capability compromises some of the originalsensor (IRST, targeting, or standoff reconnaissance) capabilities.Furthermore, the stability requirements that are needed for such anadvanced LADAR function are not characteristic of existing sensorapproaches for IRST, targeting, or stand-off reconnaissance functions.

Hence, there is a need in the art for a multi-function airborne sensorsystem that is able to combine infrared search and track (IRST),targeting and standoff reconnaissance functions on the same airbornesystem. There is also a need for a system that further provides advancedcoherent LADAR function.

SUMMARY

One or more embodiments of the present disclosure provide a sensorsystem including a first optical sub-system having a first plurality ofoptical elements, and a second optical sub-system having a secondplurality of optical elements including a first mirror. The secondoptical sub-system is configured to rotate about a first axis relativeto the first optical sub-system and the first mirror is configured torotate about a second axis substantially perpendicular to the firstaxis. The sensor system further includes a window. The first axis andthe second axis are arranged so as not to intersect each other so as toreduce a size of the window while maximizing a field of regard of thesensor system.

These and other features and characteristics of the present disclosure,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. In one embodiment of this disclosure, the structuralcomponents illustrated herein are drawn to scale. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the inventive concept. As used in the specification andin the claims, the singular form of “a”, “an”, and “the” include pluralreferents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 depicts an optical block diagram of sensor system 10 integratingthe IRST function, the standoff reconnaissance function and thetargeting function, according to one embodiment;

FIGS. 2A-2C depict a lateral view, a front view and a bottom view,respectively, of a raytrace of sensor system depicted in FIG. 1,according to one embodiment;

FIGS. 3A-3C show a ray trace of the sensor system where a coelostatmirror is rotated around rotation axis AA to produce a wide field ofregard travel in the elevation direction to locate, track or identify ascene or an object in the elevation direction, as desired in a targetingmode, according to one embodiment;

FIGS. 4A-4C show a ray trace of the sensor system where coelostat mirroris rotated around rotation axis AA and a roll gimbal in the sensorsystem including the coelostat mirror is rotated around roll axis BB toproduce a wide field of regard travel in the azimuth direction tolocate, track or identify a scene or an object in the azimuth direction,as desired in an IRST mode, according to one embodiment; and

FIGS. 5A-5C show a ray trace of the sensor system where the coelostatmirror is rotated around rotation axis AA and the roll gimbal includingcoelostat mirror 12 is rotated around roll axis BB to produce a widefield of regard pitch travel in the side direction to locate, track oridentify a scene or an object, as desired in a standoff reconnaissancemode, according to one embodiment.

DETAILED DESCRIPTION

FIG. 1 depicts an optical block diagram of sensor system 10 integratingthe IRST function, the standoff reconnaissance function and thetargeting function, according to one embodiment. Sensor system 10includes elevation (EL) and azimuth (AZ) pointing mirror or coelostatmirror (hereinafter referred to as “c-mirror”) 12, one or more windows13, fold mirror 14, afocal telescope multi-mirror arrangementfore-optics 16, wide field of view (WFOV) insert mirror 18, by-passmirror 19, derotation optical device 20, beam steering mirrors 22A, 22B,22C, laser dichroic beam splitter (B/S) mirror 23, laser module 24, beamdirection preserving device (e.g., corner cube) 25, a multi-mirrorrelayed imager or imaging optics 26, dichroic beam splitters (B/S) 28Aand 28B, auto-alignment detector (e.g., a focal plane array) 30,infrared detector (e.g., a focal plane array) 32, and visible detector(e.g., a charge coupled device or CCD) 34.

In observation mode such as standoff reconnaissance mode or IRST mode,radiation beam from an object at far field traversing one or morewindows 13 is received by c-mirror 12 which is pointed in a direction ofa scene or object being observed, i.e., pointed in a line of sightdirection (LOS). C-mirror 12 is configured to rotate around axis AA.Rotation axis AA is parallel to beam of radiation R as reflected byc-mirror 12. In one embodiment, rotation axis AA forms an angle θ ofabout 45° respective to the surface plane of c-mirror 12. Therefore, arotation of c-mirror 12 around rotation axis AA, enables c-mirror 12 toreceive radiation from a scene or object at different elevation andazimuth angles while reflecting the received radiation in generally asame direction (direction of the reflected radiation beam R). C-mirror12 directs the received radiation beam towards fold mirror 14 which inturn directs the radiation towards either afocal fore-optics 16 ortowards bypass mirror 19. In one embodiment, afocal fore-optics 16comprises an afocal three-mirror anastigmat telescope. However, anynumber (e.g., two or more) of anastigmat mirrors can be used as desired.As its name indicates, afocal fore-optics mirror arrangement 16 isafocal. Therefore, a collimated radiation beam from the object or sceneat far field received by afocal fore-optics 16 is output by thefore-optics 16 as a collimated radiation beam of generally smallerdiameter but correspondingly larger field of view. The collimatedradiation beam output by the afocal fore-optics 16 can be directedtoward derotation device 20. When the radiation beam passes throughafocal fore-optics 16, a relatively narrow field of view (NFOV) isachieved. In order to achieve a wider field of view (WFOV), afocalfore-optics 16 is bypassed using bypass mirror 19 and WFOV insert mirror18. The bypassed radiation beam is received by bypass mirror 19 whichreflects the radiation beam towards WFOV insert mirror 18. WFOV insertmirror 18 in turn is arranged to reflect the radiation beam towardderotation device 20. Therefore, in the WFOV configuration, the WFOVinsert mirror 18 and the bypass mirror 19 are moved away from the pathof the radiation beam allowing only the radiation beam output by afocalfore-optics 16 to reach the derotation device 20. While, in the NFOVconfiguration, bypass mirror and WFOV mirror are positioned in the pathof the radiation beam so that the radiation beam bypasses afocalforeoptics 16 to reach the derotation device 20.

A portion 11A of sensor system 10 including one or more windows 13,c-mirror 12, fold mirror 14, afocal fore-optics 16, WFOV insert mirror18, and bypass mirror 19 are provided on a roll gimbal (shown as axis BBin FIG. 1). A portion 11B of sensor system 10 including derotationoptical device 20, beam steering mirrors 22A, 22B, 22C, laser dichroicbeam splitter (B/S) mirror 23, laser module 24, beam directionpreserving device (e.g., corner cube or prism) 25, imager or imagingoptics 26, dichroic beam splitters (B/S) 28A and 28B, auto-alignmentdetector (e.g., a focal plane array) 30, infrared detector (e.g., afocal plane array) 32, and visible detector (a charge coupled device orCCD) 34 are mounted on a body (not shown) such as a body of an aircraft.For example, in one embodiment, portion 11B can be mounted onto astructure that can be attached to the body. The structure can be fixedrelative to the body or movable relative to the body. Line or plane CCin FIG. 1 schematically delimits the portion 11A of sensor system 10mounted on a roll gimbal and the portion 11B of sensor system 10 mountedon the fixed body.

The roll gimbal rotates around roll axis BB generally perpendicular torotation axis AA. Therefore, the portion 11A of sensor system 10including one or more windows 13, c-mirror 12, fold mirror 14, afocalfore-optics 16, WFOV insert mirror 18, and bypass mirror 19 areconfigured to rotate around axis BB relative to fixed portion 11B ofsensor system 10. Rotation axis or roll axis BB is parallel to the beamof radiation output by afocal fore-optics 16 or parallel to the beam ofradiation reflected by WFOV insert mirror 18 (in the WFOVconfiguration). The rotation around axis BB enables pointing thec-mirror in the azimuth direction or horizontal direction to enablec-mirror 12 to receive radiation from a desired object or scene.

However, due to the rotation of portion 11A mounted on roll gimbal andthus to the rotation of c-mirror 12 around axis BB, the image from thefar field object or scene is also rotated. In order to correct for therotation of the image, derotation device 20 is configured tocounter-rotate so that the image output by the derotation device is inthe same direction independent of the rotation of roll gimbal aroundaxis BB or the rotation of c-mirror around axis AA. In one embodiment,derotation device 20 is an optical prism. In another embodiment,derotation device 20 may include reflective optical elements (e.g.,mirrors) and can be, for example, an all-reflective derotation device.However, as it can be appreciated other types of derotation devices canalso be used. Furthermore, in one embodiment, derotation device 20 canbe omitted. In this case, the derotation function can be accomplishedelectronically or through image data processing. In yet anotherembodiment, derotation 20 may not be needed. For example, while thederotation function is used for the IRST function, advanced LADARscanning, and standoff reconnaissance, the derotation function may notbe needed or may be optional for targeting functions, ranging functionsand designator operation.

The radiation beam output by derotation device 20 is directed towardbeam steering mirror 22A. Beam steering mirror reflects the radiationbeam towards laser dichroic mirror 23. Laser dichroic mirror 23 isconfigured to transmit a portion of the radiation beam received frombeam steering mirror 22A towards imager 26. As will be described below,laser dichroic mirror 23 is also configured to reflect a portion of theradiation beam received from beam steering mirror 22A towards beamsteering mirrors 22B and 22C and into laser module 24. In oneembodiment, imager 26 is a focal optical system configured to form afocal image on a detector (e.g., infrared focal plane array or IR FPA)32 or detector (e.g., visible focal plane array or VIS FPA) 34, or both.In one embodiment, imager 26 is an anastigmat four-mirror system.However, imager 26 can have any number of mirrors (e.g., two or moremirrors) as desired. The radiation beam output by imager 26 is directedonto detector 32 or detector 34 depending on the radiation wavelength.Dichroic beam splitters (B/S) 28A and 28B are used to direct theradiation beam output by imager 26 depending upon the wavelength of theradiation towards either detector 32 or detector 34. For example, if theradiation beam has both an infrared component and a visible component,the infrared portion of the radiation can be directed towards detector32 while the visible portion of the radiation can be directed towardsdetector 34.

In range finder mode, LADAR mode or targeting mode, laser module 24 isused to output a laser beam for range finder, LADAR or targetingfunctions. Laser module 24 outputs a beam of radiation (shown as a solidline in FIG. 1) in any desired wavelength including infrared radiationwavelength range. The laser beam output by laser module 24 is reflectedby beam steering mirrors 22B and 22C and directed toward laser dichroicmirror 23. Laser dichroic mirror 23 reflects the laser beam towardderotation device 20 which transmits the laser beam toward afocalfore-optics 16. The laser beam exits afocal fore-optics 16 and isdirected by fold mirror 14 towards c-mirror 12. C-mirror 12 in turnreflects the laser beam towards the intended object or target.

In one embodiment, in order to track the direction of the laser beam,the laser module is also configured to emit an auto-alignment beam(shown as a dotted line in FIG. 1) that is precisely co-aligned indirection to the laser beam. In one embodiment, the auto-alignment beammay have a different wavelength than the laser beam. In one embodiment,the auto-alignment beam may have less intensity than the laser beam.Similar to the laser beam, the auto-alignment beam is reflected by thebeam steering mirrors 22B and 22C towards laser dichroic mirror 23.Laser dichroic mirror 23 is configured to transmit a portion of theauto-alignment beam when incident on one of the faces (front face) ofthe dichroic mirror 23. Therefore, in order to enable the laser dichroicmirror to reflect the auto-alignment beam towards imager 26 and stillpreserve the desired line of sight data indicative of the direction ofthe laser beam, a corner cube or prism 25 is used to reflect back theauto-alignment beam towards the back face of dichroic mirror 23. Theback face of dichroic mirror 23 is configured to reflect a portion ofthe auto-alignment beam toward imager 26. Imager 26 transmits theauto-alignment beam towards beam splitter 28A which directs theauto-alignment beam towards beam splitter 28B. Beam splitter 28A isconfigured to reflect the auto-alignment beam and transmit the infraredradiation beam output by imager 26. Beam splitter 28B is configured totransmit the auto-alignment beam towards auto-alignment detector (e.g.,auto-alignment FPA) 30 and reflect the visible radiation output byimager 26 towards detector 34. Auto-alignment detector 34 is used todetect the laser boresight direction relative to lines of sights offocal plane arrays 32 and 34.

The auto-alignment beam is used to determine the line of sight (LOS)direction of the laser, i.e., to determine the location where the laserbeam is pointing to. By using the above described optical arrangement,the auto-alignment beam “follows” the laser beam regardless of theposition of the beam steering mirrors 22B and 22C or laser dichroicmirror 23. In other words, the laser beam and the auto-alignment beamhave a common optical path from laser module 24 through imager 26.Therefore, when the laser beam is steered using beam steering mirrors22B and/or 22C and/or laser dichroic mirror 23, the auto-alignment isalso steered in the same fashion using beam steering mirrors 22B and/or22C and/or laser dichroic mirror 23. Indeed, as shown in FIG. 1, theauto-alignment beam (dotted line) and the laser beam (solid line) areboth reflected by beam steering mirrors 22B and 22C in the same fashionand thus follow substantially the same path.

However, with respect to dichroic mirror 23, the laser beam is reflectedby the front face of dichroic mirror 23 towards beam steering mirror 22Awhich in turn directs the laser beam towards derotation device 20 whilethe auto-alignment beam is transmitted through dichroic mirror 23.Therefore, in order to reflect the auto-alignment off of laser dichroicmirror 23, corner cube or prism 25 is positioned in the path of thetransmitted auto-alignment beam. Corner cube or prism 25 is configuredand arranged to reflect the auto-alignment beam onto the back face oflaser dichroic mirror 23. The back face of laser dichroic mirror 23 isconfigured to reflect a portion of the auto-alignment beam towardsimager 26. By reflecting the auto-alignment on the back face of dichroicmirror 23 and reflecting the laser beam on the front face of thedichroic mirror 23, a rotation of the dichroic mirror 23 will effect theorientation of both the laser beam and the auto-alignment beam.Therefore, the auto-alignment beam “tracks” the laser beam regardless ofthe position of beam splitters 22B, 22C and dichroic mirror 23. As aresult, by determining the position of the auto-alignment beam usingauto-alignment detector (e.g., A/A FPA) 20, the line of sight orboresight of the laser beam can be determined with considerableaccuracy.

FIGS. 2A-2C depict a raytrace of sensor system 10, according to oneembodiment. FIG. 2A is a lateral view of the ray trace of sensor system10, FIG. 2B is front view of the ray trace of sensor system 10, and FIG.2C is a ray trace of a bottom view of sensor system 10. Same characternumerals are used in FIGS. 2A-2C as in FIG. 1 to indicate correspondingoptical components. FIGS. 2A-2C shows the relative position of variousoptical elements of the sensor system 10 as well as the position of theroll axis BB and rotation axis AA of the c-mirror 12. For example, FIG.2A shows the rotation axis AA perpendicular to the plane of FIG. 2A androtation of roll axis BB substantially perpendicular to rotation axis AAwithin the plane of FIG. 2A. FIG. 2B shows the rotation axis AA withinthe plane of FIG. 2B and rotation or roll axis BB substantiallyperpendicular to rotation axis AA perpendicular to the plane of FIG. 2B.FIG. 2C shows the rotation axis AA within the plane of FIG. 2C androtation or roll axis BB substantially perpendicular to rotation axis AAwithin the plane of FIG. 2C. It is noted, however, that the roll axis BBand rotation axis AA do not intersect each other. The roll axis BB andthe rotation axis AA are separated by a distance D (shown in FIG. 2A).FIG. 2A-2C depict various optical elements including c-mirror 12, foldmirror 14 various mirrors within afocal fore-optics 16, derotationdevice 20, beam splitter 23 and mirrors within imager 26 are shown inperspective with their relative position. However, for the sake ofclarity, laser module 24 and associated beam steering mirrors 22B and22C, dichroic mirror 23, corner cube 25, beam splitters 28A and 28B, anddetectors 30, 32 and 34 are not shown in FIGS. 2A, 2B and 2C.

FIG. 2B also depicts a cylinder swept volume as circle 40. The cylinderswept volume 40 corresponds to the volume occupied by the variousoptical elements of sensor system 10, i.e., the volume swept by the pathof radiation beam. In one embodiment, as shown in FIG. 2B, a diameter ofcylinder swept volume 40 is approximately 2.5 a diameter of the aperturerepresented by dotted circle 42 approximately defined by c-mirror 12. Inother words, a cylinder diameter to aperture diameter ratio is about2.5:1 which provides a sensor system with an efficient package.

In one embodiment, the rotation axis AA of c-mirror 12 is provided inclose proximity to one or more windows 13. This enables to minimize thesize of one or more windows 13 while providing enough space for c-mirror12 to rotate around axis AA to scan in the elevation and azimuthdirections. In one embodiment, the one or more window 13 can be providedas segmented windows in the elevation direction. In one embodiment, oneor more windows 13 can be slaved to the roll gimbal in the sense thatone or more windows 13 rotate around roll axis BB in response to the useof roll axis BB in covering the desired field of regard. In oneembodiment, one or more windows 13 are not slaved in the elevationdirection in the sense that one or more windows 13 do not rotate withrotation of c-mirror 12 around axis AA. In yet another embodiment, oneor more windows 13 are not slaved in either the elevation direction orin the roll direction in the sense that one or more windows 13 do notrotate with rotation of c-mirror 12 around axis AA, nor with the rollaxis BB. In this case, the one or more windows 13 can be sized so as toprovide the desired field of regard in the elevation direction and/orthe azimuth direction. For example, in one embodiment, window 13 can bespherical so as provide a wide field of regard in the elevationdirection and/or the azimuth direction.

In one embodiment, a shortest possible coude path off of gimbal can beachieved. As can be appreciated from FIG. 1 and FIGS. 2A-2C, the coudepath corresponds to the path between the output of fore-optics 16 orfrom WFOV insert mirror 18 to derotation device 20. In one embodiment,the length of the coude path is one-half the size or diameter of theaperture. However, the coude path can be less or equal to approximatelyone-half the size or diameter of the aperture defined by the size ordiameter of c-mirror 12. By providing the shortest possible coude pathby minimizing the length of the coude path, jitter errors and/or line ofsight (LOS) errors can be minimized. Furthermore, by using a shortestpossible path off gimbal, three to four times wider field of view in theNFOV configuration can be achieved. This short coude path is in dramaticcontrast to conventional and known coude paths that are: a) much longerin total path length, b) may contain 3-5 extra flat fold mirrors forbeam re-direction within that long path length, c) pass the optical beamthrough both an elevation and azimuth bearing aperture, which greatlylimits the field of view that can be passed along the path, and d) allownumerous potential sources of line of sight and path length variationsbecause of the long total path length and the need for three to fiveadditional flat fold mirrors.

As shown FIGS. 2A-2B, two all-reflective optical assemblies are used insensor system 10: afocal foreoptics 16 and imager 26. By usingall-reflective optical assemblies for foreoptics 16 and imager 26,chromatic aberrations or restrictions can be minimized or mitigated.Also, by providing all-reflective optical assemblies, common opticalpath can be provided for all passive function and active functions thusachieving desired boresight characteristics for both functions. Inaddition, by using all reflective optical assemblies, first orderthermal sensitivity can be minimized.

FIGS. 3A-3C show a ray trace of sensor system 10 where c-mirror 12 isrotated around rotation axis AA to produce a wide field of regard travelin the elevation direction to locate, track or identify a scene or anobject in the elevation direction, as desired in a targeting mode,according to one embodiment. The same character numerals are used inFIGS. 3A-3B as in FIG. 1 and FIGS. 2A-2C to indicate correspondingoptical components. For clarity purposes some components shown in FIG. 1are omitted in FIGS. 3A-3C. For example, laser module 24 and associatedbeam steering mirrors 22B and 22C as well as 28A and 28B and detectors30, 32 and 34 are not shown in FIGS. 3A-3C. As shown in FIG. 3A,c-mirror 12 is oriented at an angle about nadir direction to stare orscan in a generally forward-downward line of sight (LOS) direction. Asshown in FIG. 3B, c-mirror 12 is oriented generally at a nadir directionto stare or scan in a generally downward LOS direction. As shown in FIG.3C, c-mirror 12 is oriented at an angle about nadir direction to stareor scan in a generally backward-downward LOS direction. These threeexamples demonstrate the ability of sensor system 10 to scan or starescan in a wide range of elevation angles. For example, when consideringthe forward direction (indicated generally by arrow 100) as a referencedirection equal to 0 deg., the range of elevation angles can betweenabout +15 deg. and about −150 deg. Therefore, a field of regard greaterthan about 165 deg. can be achieved in the elevation direction. In oneembodiment, as shown in FIGS. 3A-3C, the arrow 100 indicates generallythe forward direction is parallel to the roll axis BB.

FIGS. 4A-4C show a ray trace of sensor system 10 where c-mirror 12 isrotated around rotation axis AA and roll gimbal including c-mirror 12 isrotated around roll axis BB to produce a travel in the azimuth directionto locate, track or identify a scene or an object in the azimuthdirection, as desired in an IRST mode, according to one embodiment. Samecharacter numerals are used in FIGS. 4A-4B as in FIG. 1, FIGS. 2A-2C andFIGS. 3A-3C to indicate corresponding optical components. For claritypurposes some components shows in FIG. 1 are omitted in FIGS. 4A-4C. Forexample, laser module 24 and associated beam steering mirrors 22B and22C as well as 28A and 28B and detectors 30, 32 and 34 are not shown inFIGS. 4A-4C. As shown in FIG. 4A, c-mirror 12 is oriented at an angle tostare or scan in a generally forward-left LOS direction. As shown inFIG. 4B, c-mirror 12 is oriented to stare or scan in a generallyforward-straight LOS direction. As shown in FIG. 4C, c-mirror 12 isoriented at an angle to stare or scan in a generally forward-right LOSdirection. These three examples demonstrate the ability of sensor system10 to scan or stare objects or scenes in a wide range of azimuth angles.For example, when considering the forward direction (indicated generallyby arrow 100) as a reference direction equal to 0 deg., the range ofelevation angles can between about ±180 deg., i.e., approximately a full360 deg. Therefore, a field of regard greater than about 140 deg. can beachieved in the azimuth direction. In one embodiment, this configurationcan be used for example for IRST function.

FIGS. 5A-5C show a ray trace of sensor system 10 where c-mirror 12 isrotated around rotation axis AA and the roll gimbal including c-mirror12 is rotated around roll axis BB to produce a pitch travel in thesideways direction to locate, track or identify a scene or an object, asdesired in a standoff reconnaissance mode, according to one embodiment.Same character numerals are used in FIGS. 5A-5B as in FIG. 1, FIGS.2A-2C, FIGS. 3A-3C and FIGS. 4A-4B to indicate corresponding opticalcomponents. For clarity purposes some components shown in FIG. 1 areomitted in FIGS. 5A-5C. For example, laser module 24 and associated beamsteering mirrors 22B and 22C as well as 28A and 28B and detectors 30, 32and 34 are not shown in FIGS. 5A-5C. As shown in FIG. 5A, c-mirror 12 isoriented at an angle to stare or scan in a generally left-downward LOSdirection. As shown in FIG. 5B, c-mirror 12 is oriented to stare or scanin a generally left-backward LOS direction. As shown in FIG. 5C,c-mirror 12 is oriented at an angle to stare or scan in a generally afurther left-backward LOS direction. These three examples demonstratethe ability of sensor system 10 to scan or stare objects or scenes in awide range of pitch angles. In one embodiment, this configuration can beused for example for stand-off reconnaissance. Although, c-mirror 12 isshown staring generally in the left direction, Figures similar to FIGS.5A-5C can also be provided to show c-mirror 12 staring towards the rightdirection relative to pointing arrow 100. A field of regard of greaterthan about 75 deg. can be achieved in the pitch direction (side staringor scanning).

In one embodiment, whenever the orientation of c-mirror 12 is such thatthe sensor line-of-sight direction (LOS) is precisely parallel to rollaxis BB, for example as shown in FIG. 4B or as shown schematically inFIG. 1, a rotation of the gimbal or portion 11A of optical system 10around roll axis BB would not change the LOS direction. On the otherhand, when c-mirror 12 is oriented such that the LOS direction is at acertain angle α relative to roll axis BB (α is the angle between rollaxis BB and LOS direction), for example as shown in FIGS. 3A-3C, 4A, 4C,5A-5C (e.g., α equal to about 45 deg, about 60 deg., or about 90 deg.),a rotation of the gimbal or portion 11A of optical system 10 around rollaxis BB causes the LOS direction to sweep out a cone centered about rollaxis BB (e.g., with an angular diameter equal to about 2×45 deg., about2×60, or about 2×90). The situation where the LOS is precisely parallelto roll axis BB and thus causing the roll axis BB to not steer the LOSis called a “gimbal singularity” or “gimbal lock.” A control gain ofroll axis BB is proportional to 1/sin α. Hence, the gain of roll axis BBcan go to infinity when α is equal to 0, i.e., when the LOS is preciselyparallel to roll axis BB. From the standpoint of an automated controlsystem 90 (shown in FIG. 1) for controlling the orientation of c-mirror12, i.e., controlling the rotation around roll axis BB and rotationaround axis AA, this gimbal singularity may be problematic because noamount of rotation around roll axis BB produces any desired effect ofsteering the LOS.

As a result, in certain applications, the gimbal singularity is to beavoided. However, in an embodiment, where the gimbal singularity cannotbe avoided, for example because the gimbal singularity is within thedesired field of regard (FOR), then it may be desirable to provide athird gimbal axis TT (shown in FIG. 1). In one embodiment, third gimbalaxis TT is within the plane of c-mirror 12 and is perpendicular torotation axis AA. In one embodiment, the third gimbal axis TT resides onrotation axis AA of c-mirror 12 in that a rotation of c-mirror 12 aroundaxis AA produces a rotation of axis TT. The third gimbal axis TT can beof small angular travel (for example, less than or equal to 5 deg.). Asa result, axis TT travels around roll axis BB and avoids the gimbalsingularity.

For example, when an object being continuously tracked by movingc-mirror 12 in various directions by rotating around rotation axis AAand/or around roll axis BB and/or optional third axis TT using controlsystem 90 is projected to go close to or through the gimbal singularity,and optional third gimbal axis TT is provided with a range of angles α,for example, ±3 deg, roll axis BB is no longer used for tracking theobject within the ±3 deg. range that surrounds the gimbal singularity.Instead, rotation axis AA and third gimbal axis TT are used to continueto track the object within the ±3 deg. angular range. When, on the otherhand, the object location exceeds, for example, the 3 deg. singularity,roll axis BB is used by control system 90 in the tracking motion. Inthis case, the third axis can be gradually returned to 0 deg. and nolonger has involvement in the tracking motion. In other words, controlsystem 90 controls the tracking by rotating c-mirror 12 around thirdgimbal axis TT when an object is located closely around the singularity(e.g., within the ±3 deg. range). Otherwise, when the object is outsidethe ±3 deg. range around the singularity, control system 90 controls thetracking by rotating roll axis BB and leaving the third axis TT fixed orreturning third axis TT to 0 deg.

As it can be appreciated from the above paragraphs, in some embodiments,rotation axis AA of c-mirror 12 and roll axis BB are generally the onlytwo axes that are involved in tracking an object. However, within forexample the 3 deg. range of the singularity, it is c-mirror rotationaxis AA and third gimbal axis TT that are used. In one embodiment, forexample, within the ±3 deg. angular range, roll axis BB gain can begreater than about 19, for instance. When roll axis BB gain is greaterthan for example about 19, roll axis BB is not used by control system 90to orient or direct c-mirror 12. If a narrower angular range (e.g., ±2deg.) is selected for third axis TT to encompass the singularity, thegain for roll axis BB can increase to about 28. Therefore, the angularrange for third axis TT can be tailored so as to prevent a high gain forroll axis BB.

It is worth noting that the sensor system 10 described herein is capableof providing IRST, targeting, and standoff reconnaissance sensingfunctions utilizing either linear 1-D detector arrays operating in ascanning mode, or 2-D (e.g., square, rectangular or circular) detectorarrays operating in either staring or step-staring modes, or in anycombination of these modes. For example, IRST type sensing can equallywell be accomplished by either scanning a linear array or step-staring asquare/rectangular array. In one embodiment, target sensing can beperformed preferably by staring with a square/rectangular array becauseof the continuous coverage on the target, but target sensing can also beperformed by scanning a linear array. Standoff reconnaissance canequally well be accomplished by either scanning a linear array orstep-staring a square/rectangular array. All particular hardwarefeatures and functions that are needed for these three modes (scanning,staring, and step-staring) are provided by the sensor system describedherein.

Scanning with a linear 1-D array may require a smooth continuous motionof the line of sight (LOS) that is strictly maintained orthogonal to thelong dimension of the linear array. This can be accomplished by acoordinated operation of elevation axis TT, c-mirror axis AA, roll axisBB, derotation device 20, and beam steering mirror 22A, for example,using control system 90. Staring with a square/rectangular 2-D array,particularly during target tracking, may also require stable and smoothmotion of the line of sight (LOS) that matches that of the target andmay optionally require that a certain detector orientation be preserved.This can also be accomplished by coordinated operation of the elevationaxis TT, c-mirror axis AA, roll axis BB, derotation device 20, and beamsteering mirror 22A, for example, by using control system 90.Step-staring a square/rectangular 2-D detector array is slightly morecomplex when this mode is used for wide area search functions in IRST orstand-off reconnaissance sensing. Indeed, continuous and smooth motionsof elevation axis TT, c-mirror axis AA, roll axis BB, derotation device20 may cause motion of the 2-D array orthogonal to one of its sides.However, the array line of sight should be fixed and still in inertialspace during the frame integration time of each exposure, typicallymeasured in milliseconds. Motion of beam steering mirror 22A can be usedto momentarily cancel the scanning effects of motion of elevation axisTT, c-mirror axis AA, roll axis BB, and derotation device 20, for asmall angle (measured in milliradians) and a relatively short period oftime (e.g., few milliseconds). This operation of beam steering mirror22A is commonly termed “back-scan”, and the direction of the back-scanis held in constant orientation to the detector arrays because beamsteering mirror 22A is between derotation device 20 and passiveradiation detectors (e.g., FPAs) 32 and 34. For certain applicationswhere the continuous scan motion is very fast, and the frame integrationtime is relatively long, the back-scan angle can be somewhat large(e.g., many milliradians). For such situations, it may be advantageousto have two beam steering mirrors instead of a single mirror at thelocation of beam steering mirror 22A. This is easily accommodated withinthe sensor system, and the operation of two beam steering mirrors can beused to decrease or eliminate beam walk or wander on the fore-opticsprimary mirror. This use of two beam steering mirrors for theelimination of beam wander on the fore-optics primary mirror isprecisely what is accomplished in the active laser path by the operationof beam steering mirrors 22B and 22C.

It should be appreciated that in one embodiment, the drawings herein aredrawn to scale (e.g., in correct proportion). However, it should also beappreciated that other proportions of parts may be employed in otherembodiments.

Although the inventive concept has been described in detail for thepurpose of illustration based on various embodiments, it is to beunderstood that such detail is solely for that purpose and that theinventive concept is not limited to the disclosed embodiments, but, onthe contrary, is intended to cover modifications and equivalentarrangements that are within the spirit and scope of the appendedclaims. For example, it is to be understood that the present disclosurecontemplates that, to the extent possible, one or more features of anyembodiment can be combined with one or more features of any otherembodiment.

Furthermore, since numerous modifications and changes will readily occurto those with skill in the art, it is not desired to limit the inventiveconcept to the exact construction and operation described herein.Accordingly, all suitable modifications and equivalents should beconsidered as falling within the spirit and scope of the presentdisclosure.

What is claimed:
 1. A sensor system comprising: a first opticalsub-system comprising a first plurality of optical elements; and asecond optical sub-system comprising a second plurality of opticalelements including a first mirror and an afocal fore-optics configuredto receive radiation from the first mirror and to transmit the radiationtowards the first optical sub-system, the second optical sub-systemconfigured to rotate about a first axis relative to the first opticalsub-system and the first mirror configured to rotate about a second axissubstantially perpendicular to the first axis; wherein the first axisand the second axis are arranged so as not to intersect each other so asto maximize a field of regard of the sensor system, and wherein thefirst axis is substantially parallel to a radiation beam output by theafocal fore-optics.
 2. The system of claim 1, further comprising awindow, wherein the first axis and the second axis are arranged so asnot to intersect each other so as to reduce a size of the window.
 3. Thesystem of claim 2, wherein the window is part of the second opticalsub-system.
 4. The system of claim 2, wherein the window is part of thefirst optical sub-system.
 5. The system of claim 1, wherein the firstoptical sub-system is mounted to a body.
 6. The system of claim 5,wherein the body is a body of an aircraft.
 7. The system of claim 1,wherein the first mirror is a coelostat mirror, wherein the second axisforms an angle of approximately 45 deg. relative to the plane of thecoelostat mirror and the second axis is parallel to a beam of radiationreflected by the coelostat mirror.
 8. The system of claim 1, wherein thesecond plurality of optical elements includes a fold mirror configuredto reflect radiation received from the first mirror towards the afocalfore-optics.
 9. The system of claim 1, wherein the second plurality ofoptical elements comprises a bypass mirror and a wide field-of-viewinsert mirror configured and arranged to be movable so that radiationfrom the first mirror bypasses the afocal fore-optics.
 10. The system ofclaim 9, wherein when radiation passes through the afocal fore-optics arelatively narrow field of view is achieved and when the afocalfore-optics is bypassed using the bypass mirror and wide field of viewinsert mirror a wider field of view is achieved.
 11. The system of claim1, wherein a coude path between the second optical sub-system and thefirst optical sub-system is less than or equal to approximately half anaperture size defined by a size of the first mirror so as to reducejitter errors or line of sight errors, or both.
 12. The system of claim1, wherein a ratio of cylinder diameter swept volume size of the secondoptical sub-system to aperture size of the system is about 2.5:1. 13.The system of claim 1, wherein a rotation of the first mirror about thesecond axis provides a travel of the field of regard in an elevationdirection.
 14. The system of claim 13, wherein the field of regard ofthe optical system in the elevation direction is greater thanapproximately 165 deg.
 15. The system of claim 1, wherein a rotation ofthe second optical sub-system around the first axis provides a travel ofthe field of regard in an azimuth direction.
 16. The system of claim 15,wherein the field of regard of the sensor system in the azimuthdirection is greater than approximately 140 deg.
 17. The system of claim1, wherein the field of regard of the sensor system in a pitch directionis greater than approximately 75 deg.
 18. The system of claim 1, whereinthe first mirror is further configured to rotate around a third axissubstantially perpendicular to the second axis and in a plane of themirror, wherein a rotation of the first mirror around the third axisprevents a gimbal singularity in which a line of sight directionsubstantially coincides with the first axis.
 19. The system of claim 1,further comprises a control system configured to control an orientationof the first mirror to capture radiation from a far field object orscene.
 20. The system of claim 19, wherein the control system isconfigured to control a tracking of the object or scene by rotating thefirst mirror about a third axis substantially perpendicular to thesecond axis when the object or scene is located around a gimbalsingularity in which a line of sight direction of the object or scene iswithin a range of angles that surround the first axis, and not rotatingthe second optical sub-system about the first axis.
 21. The system ofclaim 20, wherein the range of angles is between approximately −3 deg.and approximately +3 deg. relative to the first axis.
 22. A sensorsystem comprising: a first optical sub-system comprising a firstplurality of optical elements; and a second optical sub-systemcomprising a second plurality of optical elements including a firstmirror and an afocal fore-optics configured to receive radiation fromthe first mirror and to transmit the radiation towards the first opticalsub-system, the second optical sub-system configured to rotate about afirst axis relative to the first optical sub-system and the first mirrorconfigured to rotate about a second axis substantially perpendicular tothe first axis; wherein the first axis and the second axis are arrangedso as not to intersect each other so as to maximize a field of regard ofthe sensor system; and wherein the afocal fore-optics comprises two ormore anastigmat mirrors.
 23. The system of claim 22, wherein the firstaxis is substantially parallel to a radiation beam output by the afocalfore-optics.
 24. The system of claim 22, wherein the second plurality ofoptical elements comprises a bypass mirror and a wide field-of-viewinsert mirror configured and arranged to be movable so that radiationfrom the first mirror bypasses the afocal fore-optics.
 25. The system ofclaim 24, wherein when radiation passes through the afocal fore-optics arelatively narrow field of view is achieved and when the afocalfore-optics is bypassed using the bypass mirror and wide field of viewinsert mirror a wider field of view is achieved.
 26. A sensor systemcomprising: a first optical sub-system comprising a first plurality ofoptical elements; and a second optical sub-system comprising a secondplurality of optical elements including a first mirror, the secondoptical sub-system configured to rotate about a first axis relative tothe first optical sub-system and the first mirror configured to rotateabout a second axis substantially perpendicular to the first axis;wherein the first axis and the second axis are arranged so as not tointersect each other so as to maximize a field of regard of the sensorsystem; and wherein the first plurality of optical elements comprises anoptical imager and a detector, the optical imager being configured toreceive radiation from the second optical sub-system and to relay theradiation to the detector.
 27. The system of claim 26, wherein the firstplurality of optical elements comprises a derotation device configuredto receive radiation from the second optical system and to transmit theradiation towards the optical imager, the derotation device beingconfigured to counter-rotate a beam of radiation so that an image outputby the derotation device is in a same direction independent of arotation of the first mirror.
 28. The system of claim 26, wherein thefirst optical sub-system further comprises a laser module configured toemit a laser beam and an auto-alignment beam, the laser beam beingdirected towards the first mirror.
 29. The system of claim 28, whereinthe first optical sub-system further includes a laser dichroic mirrorand a beam direction device, the laser dichroic mirror being configuredand arranged to reflect the laser beam and transmit at least a portionof the auto-alignment beam when the alignment beam is incident on afirst face of the laser dichroic mirror and to reflect at least aportion of the auto-alignment beam when the alignment beam is incidenton a second face opposite the first face, wherein the beam directiondevice is configured to direct at least a portion of the auto-alignmentbeam towards the second face of the laser dichroic mirror.
 30. Thesystem of claim 29, wherein the second face of the laser dichroic mirroris configured to reflect at least a portion of the auto-alignment beamtowards the optical imager and the optical imager is configured totransmit at least a portion of the auto-alignment beam towards anauto-alignment detector.
 31. The system of claim 30, wherein theauto-alignment beam is used to determine a line of sight of the laserbeam.
 32. The system of claim 30, wherein the laser dichroic mirror andthe beam direction device are configured so that the auto-alignment beamfollows a path of the laser beam.
 33. The system of claim 26, whereinthe first plurality of optical elements further comprises a first beamsteering mirror, wherein the beam steering mirror is configured to berotated so as to cancel a scanning motion of a line of sight of anobject or scene by a continuous rotation of the first mirror during atime period corresponding to an exposure time on the detector.
 34. Thesystem of claim 33, wherein the detector is a linear one-dimensionalfocal plane array.
 35. The system of claim 33, wherein the detector is atwo-dimensional focal plane array.
 36. The system of claim 33, whereinthe first plurality of optical elements further comprises a second beamsteering mirror and the second plurality of optical elements furthercomprises an afocal fore-optics, wherein the first beam steering mirrorand the second beam steering mirror are configured to substantiallyeliminate beam walk within a primary mirror of the afocal fore-optics.